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Hi Jim,
Jim Mora wrote:
Hi Bert, et al,
There seems to be conflicting posts on the Inet regarding Delta DC
rectification. Wye is considered 6 pulse, Delta is not well defined. I have
to ass-u-me they are both 6P rectifiers, nonetheless.
Richie's site has a (12) pulse conveniently drawn Wye and Delta in one
schematic which nicely graphically depicts the rewire in phase. (A)
"neutral" will tie B at the previous B wye HV lead and so on to complete the
triangle. It is hard to see on the xformer, but logically the Wye neutral
would be an inner winding closer to the primary, so the voltage level out is
important, consistent with the standoff from the Primary windings. Is the
voltage still 58% of the 24KV?
The previously estimated output voltage reduction factor of 58% assumed
that you used your current Wye primary with a Delta connection on the
secondary side. The secondary windings need to be connected "in series"
(from a winding sense, as can be seen in the bottom half of Richie's
schematic). Six HV rectifier stacks are connected to form a 3-phase
delta full-wave bridge (again just using the bottom half of the
schematic). Since the load will see six charging pulses per complete AC
cycle, this configuration is indeed a "6-pulse rectifier".
Since this was a mission critical, 24/7 device under oil, I am confident it
can take the stresses for Tesla intermittent duty, even if the winding
configurations are less than ideal, provided the voltage in not too high. It
is NOT, however, three pole pigs;-^)
Because of flux sharing between legs of the core, your 3-phase
transformer is more efficient (from a total iron and copper weight
standpoint) than three independent pole transformers with similar core
areas. You'll still be limited by ohmic heating of the windings, but
your transformer has huge thermal mass and can be treated (with respect
to overloading) as though it was a 24/7 distribution transformer. You've
already removed the weak link (the tube rectifiers), so you're now
limited by the peak and RMS current ratings of the diodes in your
rectifier stacks and the maximum overload current from your transformer.
As you probably know, you can estimate the rated power-handling
capability for an unknown 1-phase transformer's kVA rating if you can
measure the core's cross sectional area by using the following formula
(from Lowdon, "Practical Transformer Design Handbook"):
Est 1-Phase kVA = (F*Ac/9.6)^2
where F = line frequency
Ac = cross sectional core area (square inches)
Similarly, for your 3-phase core-type transformer (assuming balanced
loading on each phase winding), the estimated kVA rating (per phase)
will be 1.732 times the 1-phase rating:
Est 3-Phase kVA = 1.732*(F*Ac/9.6)^2
For example, a 1-phase transformer with a 4" x 4" core will have a
capacity of about 10 kVA, but a 3-phase core-type (3-leg) transformer
with the same core area/leg will have a capacity of about 17.32 kVA per
phase.
My 14" rotor has (16) .125 Tungsten through threaded brass nutted electrodes
with set screws and can comfortably do 500 - 600 BPS. I would want to design
around a .1uf cap rather than (2) in series (.05) in this case at lower
voltages and resist the temptation to twist the Variacs full tilt!
Assuming you're running with a 0.1 uF cap with resonant charging at 28
kV or so, then your bang size will be about 39 joules. At 500-600 BPS,
you'll be processing between 19.6 - 23.5 kW of tank circuit power.
You'll need to increase the diameter of your rotating tungsten
electrodes to properly handle this. You'll need at least 3/8" or 1/2"
tungsten for the rotating electrodes and your stationary electrodes
should be at least 1/2" to prevent excessive tungsten erosion/burning.
Please confirm the voltage out. I'll study more earnestly the magnetic study
documentation you sent me during the list migration.
Thanks so very much,
Jim Mora
Good luck and best wishes,
Bert
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of Bert Hickman
Sent: Tuesday, July 08, 2014 6:17 AM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Was: Raytheon turns ratio too high for sane resonant
charging, Now: more reasonabe 50:1
Hi Jim,
Oops - I just noticed an error in my previous message. The current
output (when using a delta configuration on the secondary) will increase
by Sqrt(3) or 73%, not 58% versus the previous wye configuration. Better
for rapidly (re)charging your tank cap.
Bert
Bert Hickman wrote:
Hi Jim,
Since you now have access to all the secondary leads, one option would
be to reconfigure your power supply using the existing wye (or star)
primary, and convert the secondary to delta configuration. If the
original wye-connected secondary could deliver 24 kVDC, changing the
secondary to a delta connection will result in an output that is
1/sqrt(3) times the original output, or about 58% of the original
output. The revised output will be 0.58*24 kV or about 14 kV. You can
then connect six diode strings to create a FWB DC output. This is now
in a reasonable voltage range for either direct operation or 2X tank cap
voltage multiplication using either resonant charging or a
charge-reversing (Steve Young) gap. This configuration will also provide
you with 58% higher output current (for the same transformer kVA output).
Bert
